Method and apparatus for working or finning tubing

ABSTRACT

Fins or threads are formed on the exterior surface of infinite length tubing restrained against rotation and continuously fed longitudinally to a rotating fin forming head. A mandrel initially positioned within the tubing presents a backup tool in the region of the fin forming head, the mandrel being formed from alternate sections of magnetic and nonmagnetic material and being held against axial movement by magnets surrounding the tubing and presenting alternate sections of magnetic and non-magnetic material so located that magnetic flux enters and leaves the mandrel at a plurality of locations along the length of each magnet. The fin forming head is free to move axially to avoid stressing the fin forming elements which engage and form helical threads or fins on the exterior surface of the tubing. Initially, the speed of rotation of the fin forming head is adjusted so the rate of formation of fins on the tubing equals the linear speed at which the tubing is fed through the apparatus. If the speed at which the tubing is fed changes, the finning head is free to move axially and actuates a control to either increase or decrease the speed of rotation of the head so fin forming proceeds at the new rate at which the tubing is fed. Emergency shut off means are provided to shut off critical portions of the apparatus in the event of a malfunction such as axial movement of the backup mandrel or axial movement of the rotating finning head beyond certain positions.

United States Patent [19'] Winter et al.

[451 Oct. 16, 1973 i 1 METHOD AND APPARATUS FOR WORKING OR FINNING TUBING [73] Assignee: Non-Ferrous International Corporation, New York, N.Y.

[22] Filed: May 7, 1971 [21] Appl. No.: 141,249

[52] US. Cl 72/78, 72/209, 72/370, 335/266 [51] Int. Cl. B21h 3/12 [58] Field of Search 72/283, 209, 208, 72/370, 479, 78; 335/266 [56] References Cited UNITED STATES PATENTS 3,379,042 4/1968 Brown 72/78 3,167,176 l/1965 Hill et al 72/283 2,600,254 6/1952 Lysobey 72/ 198 1,474,124 11/1923 Steinecke 72/209 3,149,255 9/1964 Trench 335/266 2,100,587 11/1937 Chalker 91/32 FOREIGN PATENTS OR APPLICATIONS 943,044 5/1956 Germany 72/283 962,489 7/1964 Great Britain 72/78 [5 7] ABSTRACT Fins or threads are formed on the exterior surface of infinite length tubing restrained against rotation and continuously fed longitudinally to a rotating fin forming head. A mandrel initially positioned within the tubing presents a backup tool in the region of the fin forming head, the mandrel being formed from alternate sections of magnetic and nonmagnetic material and being held against axial movement by magnets surrounding the tubing and presenting alternate sections of magnetic and non-magnetic material so located that magnetic flux enters and leaves the mandrel at a plurality of locations along the length of each magnet. The fin forming head is free to move axially to avoid stressing the fin forming elements which engage and form helical threads or fins on the exterior surface of the tubing. Initially, the speed of rotation of the fin forming head is adjusted so the rate of formation of fins on the tubing equals the linear speed at which the tubing is fed through the apparatus. 1f the speed at which the tubing is fed changes, the finning head is free to move axially and actuates a control to either increase or decrease the speed of rotation of the head so fin forming proceeds at the new rate at which the tubing is fed. Emergency shut off means are provided to shut off critical portions of the apparatus in the event of a malfunction such as axial movement of Claims, 9 Drawing Figures PATENTEB OCT 1 61973 .SHEET H 0F 5 INVENTORS May/4R0 M. W/NTER W/LU/IM F firm/w WM 9 W T0 E KS METHOD AND APPARATUS FOR WORKING OR FINNING TUBING This invention relates to a method and apparatus for performing a work operation on an indefinite lenth of nonmagnetic material tubing. More particularly, the invention relates to a method and apparatus for forming fins on the exterior surface of an indefinite length of small diameter tubing wherein a backup tool within the tubing is connected to a floating mandrel held against axial movement by magnetic forces.

In addition, the invention relates to an improved magnet arrangement for holding a floating mandrel disposed within non-magnetic or slightly magnetic material tubing against axial movement while moving the tubing axially and performing a work operation on the exterior of the tube with collapse of the tube resisted by a backup tool held by the mandrel. In the preferred embodiment, the mandrel is formed from alternate magnetic and non-magnetic material sections, is held against axial movement by a plurality of electromagnets each including plural pole pieces surrounding the tubing and mandrel, and the mandrel holds a backup tool or anvil within the tubing which prevents crushing or other undesirable deformation of the tubing at a location where a tube finning head, which rotates about the tube, engages the tube to roll a form such as a deep thread or an annular or helical fin on the exterior of the tube. However, instead of forming fins or threads, the apparatus can be used to form any type of enhanced surface on a tube.

Further, the invention relates to a method and apparatus for rolling helical fins on the exterior surface of a tube, or otherwise enhancing the surface of the tube, wherein the tube is advanced at a predetermined constant rate, the forming rolls are mounted on a forming head which rotates about the tube with the forming rolls in engagement with the tube, the forming head is on a floating support which permits axial movement of the head relative to a normal position, and control means responsive to movement of the head from the normal position either increases or decreases the speed of rotation of the head so the rate at which fins are formed on the tubing corresponds to the feed of the pedestal BACKGROUND OF THE INVENTION It is well known that fins or other surface deformations which increase the surface area of the outside of tubing improves the heat transfer characteristics of the tubing, and hence, tubing with fins formed thereon is extensively used in heat exchangers. In the past, it has been possible to form fins only on relatively short finite lengths of tubing. The usual arrangement is to provide an elongated mandrel which is fixed at one end, position the tube to be finned on the mandrel and then, form fins on the tube as the tube is moved axially relative to the tube-forming head and mandrel. Such prior apparatus does of course require that the mandrel have a length at least as long as the length of the tube on which the fins are formed. The operation of such prior art apparatus is of course discontinuous in that it is necessary to start the finning operation at one end of the tube and stop the operation at the other end when the length of tubing is finned. Then, it is necessary to load another length of tubing into the apparatus and proceed in the same manner. Because of the necessity of the start-stop operation, finned tubing is quite expensive to produce. In addition, the discs of the fin forming head frequently break as they are initially forced against the tubing. It is also known that the finning head, when it is initially moved into engagement with the tube, does not form a fin of proper depth at the end of the tube. Hence, in many instances, at least several inches of each length of tubing finned by the prior art methods and apparatus must be cut off and discarded.

In addition, some heat exchangers require substantially more tubing than the length that can be finned in accordance with the known methods and apparatus. Hence, it is frequently necessary to connect several lengths of existing finned tubing together when the heat exchanger is assembled. Such connections are of course undesirable since most failures occur at a joint or coupling, and the cost of the heat exchanger is materially increased because of the expense of forming the joints and couplings.

These prior art problems are completely eliminated by applicants apparatus which provides for forming fins on an indefinite length of tubing continuously. The only restriction on the length of tubing that can be finned is the length that is avaible for processing in accordance with this method and apparatus. It is contemplated, however, that the tube finning apparatus of this invention could be used in conjunction with a seamless tube-forming mill where the tubing from the mill is fed directly to the fin forming apparatus. In this instance, there is no limit to the length of the tubing which can be finned, and so long as the mill produces tubing, fins can be formed on the tubing continuously.

In addition, where fins are formed onexistingtubing, tubing coils of 500 feet and longer may be u s e d to maintain the number of start-stop operations at a minimum.

SUMMARY OF THE INVENTION In accordance with the method and apparatus of this invention, small diameter (%-l inch inside diameter) non-magnetic material tubing is continuously fed from a coil or other supply to a finning head and fins are formed on the exterior surface of the tubing in a continuous operation. In the preferred embodiment the tubing is fed from a coil containing perhaps 500 feet of the tubing. The tubing is uncoiled, straightened, fed to the finning head, and after the finning operation is completed, is either recoiled or the straight sections leaving the finning head, are cut to a desired length. When tubing from a coil is exhausted, it is of course necessary to feed the forward end of another coil of tubing through the apparatus. Where the method and apparatus of this invention is used in conjunction with a tube forming mill, the tubing is of course fed continuously to the tube finning apparatus.

The fin forming head is of the type which engages the outside of the tubing and rotates around the tube at a location where the anvil or backup tool connected to the mandrel is located within the tube. The fin forming head includes several grooved or threaded opposed rollers which are forced into the outside surface of the tube to extrude or roll helical fins of the desired height on the tube. During this rolling process the tube is compressed radially against the backup tool connected to the mandrel. Simultaneously, the tube is moved axially relative to the head so a continuous helical fin is formed on the outside of the tube. As a result of compression of the tube and the axial movement of the tube substantial axial forces are exerted on the backup tool and tend to pull the tool axially with the tubing. Such displacement of the tool is prevented by the mandrel to which the tool is connected and which is held against axial movement by electromagnets encircling the tubing and mandrel at a location upstream from the fin forming head. A mandrel and electromagnets of unique construction which hold the mandrel against endwise movement form a significant feature of this invention.

During the initial operation of starting the fin forming apparatus, it is of course necessary to initially insert the mandrel into the tube. The mandrel and backup tool are then properly positioned within the tube with the mandrel magnetically aligned with plural magnets that hold the mandrel against endwise movement and with the backup tool correspondingly in transverse alignment with the forming rolls of the finning head. When the mandrel and backup tool are inserted in the tube a lubricant is also forced into the interior of the tube so friction between the backup tool and the tubing at the finning head is maintained relatively low. In some instances, a swivel joint may be provided between the magnetically held portion of the mandrel and the backup tool. This swivel joint allows the backup tool to rotate in response to the progressive helical deformation of the tube against the backup tool as the finning head rotates and forms fins on the tube, without rotating the mandrel.

In the preferred embodiment, the finning head is driven by a hydraulic motor the rotational speed of which is controlled in response to the position of the floating finning head relative to its normal axial position. When the head is moved from its normal position in the direction of travel of the tubing, a sensing device operates to increase the flow of luid to and correspondingly to increase the speed of rotation of the hydraulic motor which rotates the head. If the head is rotating too fast and tends to climb along the tube so it moves in a direction opposite to the direction of travel of the tube, the sensing device operates to decrease the speed of rotation of the head. While a hydraulic motor is used to control the speed of the head, other variable speed motors such as electric motors can also be used. By virtue of this arrangement, it is assured that the linear speed at which the tube is fed is always approximately the same as the axial rate at which fins or threads are formed on the tube and by virtue of the axially movable forming head, there are no appreciable axial stresses between the tube and the threads or teeth of the forming rollers.

OBJECTS OF THE INVENTION In view of the above, an object of this invention is a method and apparatus for forming a helical fin on tubing in a continuous manner by engaging the exterior surface of the tubing with a fin forming tool and wherein a backup tool initially placed within the tube is magnetically held against axial displacement by plural electromagnets which extend around the outside of the tube.

Another object is a method and apparatus for continually forming helical fins on the exterior surface of a tube of nonmagnetic material of indefinite length wherein a backup tool is disposed within the tube in opposed relation to a rotating finning head including rollers which are pressed against the outside of the tube,

and the backup tool is held against axial movement by plural magnets.

Another object is a unique mandrel and magnet arrangement wherein the mandrel is comprised of plural sections vof magnetic and non-magnetic material secured together in end to end relation and in which the magnet has hollow alternate rings of magnetic and nonmagnetic material providing axially spaced pole pieces, which rings and alternate sections co-operate to provide a substantial magnetic force resisting axial movement of the mandrel during a continuous work operation on an axially moving indefinite length of nonmagnetic material tubing.

A further object is a unique electromagnet and mandrel construction where each of a plurality of electromagnets has plural pole pieces energized by a single coil, the mandrel is within tubing of non-magnetic material and is comprised of alternate sections of magnetic and non-magnetic material so spaced apart that plural magnetic forces are exerted by each electromagnet to resist axial movement of the mandrel with the tube, and the plural pole pieces and'mandrel sections co-operate'to provide a relatively axially short magnet and mandrel assembly.

A further object is a rotating thread or fin forming head which is freely axially movable within predetermined limits, with the tubing on which the helical fins are formed, and in which control means rotate the head at a rate related to the pitch of the threads formed on the tube and the speed of advance of the tube, such that the position of the head does not deviate significantly from a normal centered axial position, while axial stresses between the tube and the head are substantially eliminated.

A further object is a method and apparatus for continuously forming fins or threads on the exterior surface of an indefinite length of non-magnetic material tubing wherein a finning head is free floating axailly, the tubing is fed to the finning head at a constant rate and is restrained against rotation, the finning head is rotated at a speed related to the pitch of the thread formed on the tubing and the speed of travel of the tubing to maintain the finning head in a normally centered position on its floating support, and the speed of rotation of the head is automatically increased or decreased in response to movement of the head axailly from its normal position so the rate at which fins are formed is adjusted to correspond to the speed at which the tubing is fed.

Numerous other objects, features and advantages of the method and apparatus of this invention will become apparent with reference to the accompanying drawings which form a part of this specification, and in which:

FIGS. la and 1b show the tube working apparatus of this invention diagrammatically in front elevation with portions thereof cut away for purposes of explanation;

FIG. 2 shows the manner in which FIGS. la and 1b are related to each other;

FIG. 3 is an enlarged view in vertical section with portions broken away showing the electromagnets, mandrel swivel and a fin forming element;

FIG. 4 is a partial view in section taken along line 4-4 of FIG. 3

FIG. 5 is an enlarged front elevational view in partial section and with portions thereof removed, of the floating finning head assembly according to this invention;

FIG. 6 is a top plan view of the finning head with the dust cover removed, showing the control for regulating its speed of rotation, and the automatic shut off for stopping the apparatus in the event that the finning heads moves too far axially;

FIG. 7 is a front elevational view in partial section showing additional details of the finning head;

FIG. 8 is an end elevational view of the finning head with portions thereof in section and portions removed for purposes of explanation.

Referring now to the drawings in detail and in particular to FIGS. la and 1b, there is shown the apparatus, in accordance with this invention, for continuously forming a helical fin on the exterior surface of indefinite length tubing. At one end of apparatus 1 is a coil support and pay-off 2 adapted to receive a coil 3 of tubing to be finned by the apparatus, and from which the tubing is fed to the apparatus.

As shown at FIG. 1a and 1b, the various components of the apparatus are mounted on suitable supports in the form of frames or tables which rest on the floor 4 of the building where the apparatus is installed. Apparatus 1 includes first and second feed roller and straightening sections 5 and 6, a magnet section 7, a third roller section 8, a fin forming station 9, and a cutoff and discharge station 10.

First feed roller and straightening section 5 is located adjacent coil payoff 2. Roller section 5 includes a pair of guide rollers 11 mounted for rotation about horizontal axes, a pair of pinch rollers 12 which perform a driving function and which are mounted for rotation on vertical axes, and a plurality of staggered straightening rollers 13 mounted for rotation on vertical axes and which function to straighten the tubing 18 in a vertical plane. It will be observed with reference to rollers 12, for example, that the periphery of each roller is concave so a substantial area of the exterior of the tubing is engaged by each roller.

From roller section 5, the tubing 18 passes to roller section 6 which includes a plurality of upper rollers 14 and lower rollers 15. There are also pinch rollers 16 and 17. Rollers 14 are staggered relative to rollers 15 .and these rollers are all mounted for rotation about horizontal axes. The rollers 14 and 15 co-operate to straighten the tubing in a horizontal plane, whereas rollers 16 and 17 are drive rollers to propel the tubing. The drive rollers 12, and the drive rollers 16 and 17 are each driven at a constant speed so tubing passing through the nips of these rollers are driven longitudinally through the apparatus at a predetermined speed. Guide rollers 11, straightening rollers 13, and straightening rollers 14 and 15, can also be driven, if necessary, to propel the tubing. Each of the rollers 11-17 is adjustably mounted to permit precise adjustment of the location of the roller and the size of the nip between the drive rollers. Correspondingly, the pressure exerted on the exterior of the tubing by the several rollers '11-17 can be accurately adjusted. An additional advantage of the adjustable rollers is that these rollers can be adjusted to accomodate several different sizes of tubing on which fins are to be formed, for example, in the range of 56-1 inch. Tubing fed through the apparatus travels along the path indicated by the center line 19, and the various portions of the apparatus are aligned relative to this centerline.

Magnet section 7 is comprised of eight identical electromagnets 20-27, each of which is generally cylindrical and has a centrally located opening therethrough. Magnets 20-27 are mounted with the axes of their openings aligned with the nips of the roller pairs ll-17. With the electromagnets in aligned end-to-end relation, a passage is defined through which tubing 18 moves axially during the operation of the appartus. Electromagnets 20-27 will subsequently be described in detail.

From magnet section 7 tubing 18 passes through the nips of third roller section 8 which includes roller pairs 30-32. These roller pairs are pinch rollers identical to those previously described in section 6. Each roller of the roller pairs 30-32 is driven, and these rollers function to maintain the portion of the tubing within magnet assembly 7 substantially straight as well as to prevent any rotation of the tubing about its axis as a result of the rotating fin forming rollers at fin forming station 9.

From roller section 8, the tubing travels to fin forming station 9 where helical fins are formed on the exterior surface of the tubing. From fin forming station 9 the tubing travels to cutoff and discharge station 10 where outfeed pinch rollers 34 are provided to convey the tubing away from apparatus 1. At outfeed station 10, a motor driven cutoff saw assembly 35 is provided. Cutoff assembly 35 is mounted on a support 36 for axial movement with the tubing at the same rate as the linear speed that the tubing travels through the apparatus. Cutoff assembly 35, when actuated, moves cutoff saw 37 into engagement with the tubing and simultaneously, the cutoff assembly 35 is moved axially so the tubing is out while it is moving.

In instances where it is desired to make coils or finned tubing of an indefinite length, cutoff assembly 35 is not used but instead, suitable apparatus (not shown) for coiling the tubing after it is finned, is substituted for the cutoff assembly.

Coil support and payoff 2 provides for feeding tubing 18 continuously to the apparatus from the coil 3. Coil payoff 2 includes a support or table 40 having upstanding rollers 41 to feed tubing from the coil. Advantageously, table 40 is provided with roller bearings (not shown) so the coil is free to rotate to feed tubing to roller section 5 at the inlet end of the apparatus.

As shown at FIGS. 1a and lb, an elongated mandrel 44 extends from a location beyond fin forming station 9 through magnet assembly 7. As shown at FIG. 3, mandrel 44 is within tubing 18 and is restrained against axial movement with the tubing by magnet assembly 7.

With reference to FIGS. 10 and lb, it will be seen that mandrel 44 is cylindrical and is comprised of an armature portion 56 including eight armature sections 46-53 which are located respectively within magnets 20-27. Connected to the armature portion 56 is an' anvil or tool portion 55. Slightly to the left of section 53 (FIG. 1b) is a swivel connection 54 which can be used to join anvil portion 55 of the mandrel with armature portion 56. The anvil portion 55 can also be directly connected to armature portion 56 in which case the swivel connection 54 is not used and the armature is rigid.

FIG. 3 shows the mandrel in greater detail, and also shows the details of the electromagnets 26 and 27. With reference to FIG. 3, it is apparent that mandrel 44 is located within tube 18 and that the sections 52 and 53 of the mandrel extend through the electromagnets 26 and 27 respectively. Armature sections 46-53 of the mandrel are identical and each includes seven pieces of magnetic material 57-63 separated from each other by nonmagnetic material spacers 64-69. The end pieces 57 and 63 are longer axially than the intermediate sections 58-62. The several sections 46-53 are separated from each other respectively by nonmagnetic magnetic material separators 70 which are substantially longer than the spacers 64-69 and are essentially equal in length to the spacing between adjacent ones of the electromagnets, for example, the electromagnets 26 and 27 shown at FIG. 3.

As shown at FIG. 3, for magnets 26 and 27, each of the magnets -27 are identical. Magnet 27 includes a cylindrical body 71 with a side wall 72 which has inwardly extending integral end flanges 73 and 74 respectively, which provide an annular space 75 for an electromagnetic coil 76. The inside of the coil and the openings through end flanges 73, 74 provide a smooth bore 77 to receive eight magnetic material pole piece sleeves 78-85 separated respectively by non-magnetic material spacer sleeves 86. End pole piece sleeves 78 and 85 are longer axially than the intermediate pole piece sleeves 79-84. The several magnetic and nonmagnetic material sleeves are retained in the bore 77 against axial movement by annular clamp rings 87 at the opposite ends of the magnet and which are secured to the ends of the magnet with bolts 88 to maintain the sleeves in tight abutting relation to each other and against axial movement within bore 77.

With reference to FIG. 3, it will be seen that in the working position of magnet 27 and mandrel 44, magnetic material piece 57 of the mandrel is in opposed relation to end pole piece 78 of the magnet and that the right hand end of piece 57 extends axially through the first spacer sleeve 86 to a position where it slightly overlaps magnetic material sleeve 79. Similarly, magnetic material section 63 of the mandrel is in opposed relation to end pole piece 85 of the magnet and the left hand end of section 63 extends axially through a spacer sleeve 86 and to a position so it slightly overlaps pole piece sleeve 86. It will also be seen that each of spacers 64-69 of the mandrel has a width slightly less than the width of pole'piece sleeves 79-84 and that in the working position of the magnet and mandrel, these spacers and pole pieces are aligned with each other in a plane perpendicular to the axis of the mandrel. Magnetic material pieces 58-62 are each of the same width, which is slightly greater than the width of spacer sleeves 86 and, with the magnet and mandrel in the working position of FIG. 3, the several magnetic material sections 58-62 are transversely aligned with spacers 86.

By virtue of the construction of magnet 27, wherein pole pieces 78-85 are spaced apart from each other, and magnetic material sections 57-63 of mandrel 44 are spaced from each other and have their several magnetic material sections positioned substantially between the several pole piece sleeves, magnetic flux 90 generated by energizing the coil 76, (the coil energizes magnet 27 in an axial direction) enters and leaves each of the magnetic material sections 57-63 of the mandrel. In the arrangement shown where magnet 20 has eight pole piece sleeves and mandrel section 53 has seven magnetic material pieces, flux 90 enters and leaves the magnet at seven axially spaced locations to enhance the resistance of the mandrel to axial displacement from the aligned FIG. 3 position within the magnet. It has been found that this arrangement using eight pole piece sleeves in the magnet and seven magnetic material sections on the portion of the mandrel within the magnet provides five times more resistance to axial displacement of the mandrel than where the mandrel section 46 is a continuous length of magnetic material and the magnet has only two pole piece elements.

Each of magnets 20-27 is identical to the magnet 27. In addition, each of sections 46-53 of the mandrel is identical to the section 53. There is a separator between each of the sections 46-53. In the preferred embodiment, pole piece sleeves 78-85 of the magnet are formed from iron or steel with good magnetically conducting properties, whereas spacers 86 are formed from non-magnetic materials, such as copper or brass. As previously explained, pole piece sleeves 78-85 are held in abutting relation to spacers 86. With regard to mandrel 44, each of magnetic material sections 57-63 is formed from iron or steel with good magnetically conducting characteristics, whereas spacers 64-69 and separators 70 are formed from copper or brass. The several magnetic material sections, spacers, and separators are secured together, for example, by brazing, so that armature section 56 of the mandrel is rigid.

Secured to the right hand end of magnetic material section 63 of the mandrel is a brass spacer 93 which extends to the right of magnet 27. Adjacent the outside of tube 18 in transverse alignment with non-magnetic material spacer 93 is a magnetic material proximity sensing device 94. In the event that mandrel 44 moves axially to the right as a result of a malfunction in the apparatus, magnetic material piece 63 also moves to the right and sensing device 94 senses the presence of the magnetic material. The output from sensing device 94 can be used to sound an alarm or alternatively, can be used to operate a master circuit breaker or control to shut down the apparatus.

As previously explained, armature section 56 of the mandrel can be connected to too] section 44 of the mandrel by a swivel joint 54. Advantageously, swivel joint 54 is located downstream from magnet assembly 27, yet is a substantial distance upstream from fin forming station 9. So locating the swivel joint provides sufficient time to shut down the apparatus before the slightly larger diameter swivel joint 54 reach the fin forming apparatus where it could cause damage, should the mandrel 44 move axially. As shown at FIG. 3, the swivel joint includes a threaded sleeve 96 into which one end of the tool portion 55 of the mandrel is threaded and is secured against unthreading by a set screw 97. The other end of the sleeve 96 has a smaller diameter opening 98 through which a threaded stud 99, having an enlarged head 100 extends. Armature portion 56 of the mandrel is threadedly connected to stud 99 and is secured to the stud against relative rotation by a set screw 101. Ball bearings 102, between the tapered axially facing surface 103 of sleeve 96 and the opposing surface of head 100 provide a low friction bearing connection which permits tool portion 55 of the mandrel to rotate freely relative to armature portion 56, but joins the portions against axial separation. Opening 98 is made sufficiently large that armature portion 56 of the mandrel can also tilt slightly relative to tool portion 55.

As shown at FIG. 4, armature portion 56 of the mandrel has a diameter which is only slightly less than the inside diameter of tube 18. Similarly, the outside diameter of tube 18 is only slightly smaller than the inside diameter of the opening 105 of the several pole piece and spacer sleeves of each magnet. By virtue of this arrangement, the air gap between the several pole piece sleeves of the magnet and the magnetic sections of the mandrel is maintained at a minimum to assure sufficient attraction between the mandrel and magnet that axial displacement of the mandrel is effectively prevented. There is, however, sufficient clearance between the mandrel and the inside of the tube, and opening 104 through the magnets and the outside of the tube, that the tube moves through the magnets and over the mandrel with very little frictional resistance. As shown at FIG. 1a, the coils of the magnets 20-26 are engergized with direct current conducted by wires 105 and 106 connected to a suitable direct current power supply. The coils of the several magnets are connected across wires 105 and 106 so the coils are in parallel. The wires are connected to the coils at junction boxes 107 adjacent each magnet. A switch 108 in wire 106 is provided to control energization of the coils.

THE FIN FORMING STATION As shown at FIGS. lb, and 6, fin forming station 9 includes a fin forming mechanism 110 having a bore 111 therethrough and into which the tool end of tool portion 55 of the mandrel extends. A dust cover 112 extends over the mechanism 110.

FIGS. 5 and 6 show the details of fin forming mechanism 110. With reference to FIGS. 5 and 6, it will be seen that fin forming head 113 is at the left hand end of the assembly and is secured to a hollow drive sleeve 114. Sleeve 114 is secured to a thick walled hollow drive shaft 115 mounted for rotation on a support pedestal 116 by bearings 117 and 118. As is apparent with reference to FIG. 5, sleeve 114 and drive shaft 115 mount head 113 in overhanging cantilever fashion for rotation with the sleeve and shaft and against axial movement relative to the support pedestal 116.

Pedestal 116 is mounted for horizontal movement on a pair of spaced apart cylindrical slide bars 120 secured to base 121 against endwise movement. There are two bars 120 which are parallel to each other and are equally spaced on each side of a vertical plane passing through the center line of the fin forming assembly. Suitable bearings 122 are provided between pedestal 116 and slide bars 120 to maintain friction at a minimum and provide for free floating axial movement of finning head 113 with pedestal 116, relative to base 121.

Sleeve 115 is elongated and extends beyond end 123 of pedestal 116. Keyed to the end of hollow drive shaft 115 is a sprocket pulley 127 which is driven by a toothed timing belt 128 that connects the pulley 127 to a toothed drive pulley or drum 129 (FIG. 1b) on a variable speed hydraulic motor 130 located below the fin forming apparatus. Sprocket pulley 127 has side flanges 131 which prevent timing belt 128 from moving axially relative to this pulley. Drive pulley 129 is elongated axially and timing belt 128 moves axially along the drive pulley when the finning head moves axially so the head is continuously driven.

By virtue of this arrangement, pedestal 116 and fin forming head 113 can freely move as a unit along slide bars 120 in an axial direction relativeto a tube fed through the apparatus while head 113 is driven by hydraulic motor 130.

FIGS. 5-8 show the details of fin forming head 113. With reference to FIGS. 7 and 8, fin forming head 113 includes four fin forming rollers 137 (only two of which are shown at FIG. 7) in equally spaced circumferential relation on the head. Each roller 137 is mounted for rotation on a shaft 138 supported by a support block 139. The axis of shaft 138 is tilted slightly relative to a plane including the axis of tube 18 so rotation of head 113 while rollers 137 are in engagement with the exterior surface of the axially moving tube causes helical fins to be formed on the outside of the tube. Support blocks 139 are mounted for limited radial movement in slots 140 of head support sleeve 114. Each block 139 is normally urged outwardly away from tubing 18 by a spring 142. Cam faces 143, at the outer surface of block 139, are provided to move the block inwardly so fin forming rollers 137 are forced into the outside surface of tube 1 18.

Blocks 139 are moved inwardly by a plurality of cam rollers 144 mounted on an actuating ring 145 arranged to move axially relative to support sleeve 114. Actuating ring 145 is moved axially while it is rotating with the head by a yoke 146 which engages in a peripheral groove 147 of ring 145. As shown at FIG. 7, ring 145 is in its forward position where rollers 144 cam support blocks 139 inwardly to force rollers 137 into engagement with the exterior surface of the tube for form a deep thread of helical fin on the surface of the tube. In a retracted position (not shown) of ring 145 rollers 144 and the ring are moved axially so rollers 144 engage the inner cam faces 143 to allow support blocks 139 to move outwardly away from the surface of the tube under the action of the springs 142.

With reference to FIGS. 5 and 7, yoke 146 is moved to correspondingly move actuating ring 145 by the action of a hydraulic cylinder 148 having its piston rod pivotally connected to the upper end of yoke arm 149. The outer end of cylinder 148 secured to a bracket 150 fixed to pedestal 116 so the cylinder and bracket move with the pedestal when head 113 moves axially. An intermediate portion of yoke arm 149 is pinned as at 151 to an arm 152 adjustably secured to bracket 150. By virtue of this arrangement, it will be apparent that when cylinder 148 is elongated, actuating ring 145 is retracted and fin forming rollers 137 are away from the surface of tube 18. When cylinder 148 is contracted to the position shown at FIGS. 5 and 7, actuating ring 145 moves forward and rollers 144 cam fin forming rollers 137 against the outside surface of the tube with sufficient force to form helical fins or threads on the surface of the tube.

HEAD SPEED CONTROL As previously explained, the rollers of roller sections 5, 6 and 8 feed tubing 18 through the apparatus at a constant speed, i.e., the drive rollers of each of these roller sections is driven at a constant speed. As a result of unexpected variables, however, for example, unexpected slippage, it has been found that the tubing is really not fed at a constant speed even though the drive rollers rotate at a constant speed. As a result of the freely axially movable floating pedestal 116 on which finning head 113 is mounted, slight changes in the speed at which tubing is fed through the apparatus cause the head 113 to move axially with edestal 116 so no appreciable axial forces are exerted on the deep fin forming threads of the fin forming rollers. Since the extent of permissible axial movement of the pedestal 1 16 and head 113 is limited by the length of the bearing bars 120, the speed of rotation of finning head 113 is adjusted to maintain the head and pedestal between the front and rear limits of travel of the pedestal. This speed control is accomplished by changing the speed of rotation of the hydraulic motor 130 which rotates the finning head, and the speed of the motor is advantageously controlled in response to movement of the pedestal toward its front and rear limits of travel.

During normal operation of the apparatus, finning head 113 is rotated at a speed such that the rate at which threads are formed along the length of the tube equals the linear speed at which the tube if fed through the apparatus (thread forming rate equals thread pitch times speed of rotation of head 113). If the tube is fed at a linear speed greater than the rate at which the threads or fins are formed on the tube 18, finning head 113 will move pedestal 116 axially on the slide bars 120 in the direction of travel of the tube. Then, it is necessary to increase the speed of rotation of head 1 13 so the fins are formed at a faster rate on the tube. Conversely, if head 113 is rotating too rapidly, the head will tend to climb along the tube and move in a direction toward roller section 8 of the apparatus. It is then necessary to decrease the speed of rotation of the head so the rate of formation of fins on the tube precisely equals the lincar speed of the tube.

Such regulation of the speed of motor 130 and correspondingly, the speed of head 113, can be accomplished manually, or automatically.

Automatic control of the speed of rotation of the finning head is accomplished by automatically adjusting flow control valve 160 in response to the axial position of finning head 113 and pedestal 116 on the slide bars 120. As shown at FIG. 6, flow control valve 160 is connected in the hydraulic system 161. Hydraulic system 161 includes a pump 162 driven by a motor 163. Motor 163 can be of the variable speed type or alternatively, a variable speed drive can be used to couple the motor to the pump so the pressure and flow of hydraulic fluid from the pump can be regulated. Pressurized fluid from pump 162 flows to accumulator 163 and then to the inlet 164 of hydraulic motor 130 which rotates finning head 113. Then the hydraulic fluid flows through the outlet 165 of motor 130 to inlet 166 of valve 160. Hydraulic fluid passes through valve 160 to the valve discharge 167 and thence through the return flow line 168 to resevoir 169 which is connected to the inlet 170 of pump 162. There is also a safety valve 171 in a return flow line 172 which connects between accumulator 163 and inlet 164 of motor 130. In the event ofa malfunction where it is desired to stop motor 130, valve 171 is opened thereby bypassing hydraulic fluid from the accumulator directly back to the reservoir 169. In addition, there is a hydraulic circuit 173 connected in parallel across valve 160 and which includes a manually operable flow control valve 174 which is manually operable by manipulation of lever 175 to control the speed of motor 130 by regulating the flow of hydraulic fluid from the discharge 165 of the motor.

Flow control valve 160 is of the type shown schematically at FIG. 6 wherein the flow of hydraulic fluid through the valve is directly proportional to the position of the valve within its housing. As shown at FIG. 6, valve 160 includes a housing 176 and a flow control element 177 in the form of a spool. Connected to flow control element 177 is an operating rod 178 provided with a cam roller 179 at its outer end. Roller 179 engagesa tapered plate type cam 180 secured to pedestal 116 by studs or bolts 181. Cam plate 180 has a tapered or slanted cam face 182 engaged by roller 179 and which face resists movement of the spool toward the cam under the action of spring 183. Plate cam 180 is provided with elongated slots 184 which facilitate positioning the cam in a desired position axially along pedestal 116.

The action of valve under the control of cam 180 is such that when tubing 18, which travels in the direction of arrow 185, is travelling faster than the rate at which fins are formed on the tubing by the rotating head 113, head 113 and pedestal 116 move in the direction of arrow 185. As a result, cam 180 moves in the same direction thereby allowing spring 183 to further open the valve by maintaining cam roller 179 in engagement with face 182. As a result, hydraulic fluid can flow faster from outlet of motor 130, and correspondingly, the speed of motor 130 and head 113 driven thereby increases. Such increase in the speed of rotation of the head causes fins to be formed at the same rate as the tubing is fed through the apparatus, and correspondingly, no further axial movement of the head occurs. I

To initially adjust the position of pedestal 116 relative to valve 160, manual control valve 174 can be regulated by manipulating the lever 175. Under some circumstances it may be desirable to control the speed of motor 130 wholly manually. Where complete manual control is desired, valve 160 is disabled so all the hydraulic fluid flowing through the motor discharges through the manual control valve 174. Under these conditions complete manual control of the speed of rotation of motor 130 is obtained.

As shown at FIG. 3, the fins or teeth of fin forming rollers 137 form a deep thread or helical fin on the exterior surface of tube 18. The action of the fin forming rollers 137 is to compress the tube radially into engagement with tool portion 55 of the mandrel, thereby simultaneously roll forming or extruding fins on the exterior surface of the tube and reducing the inside diameter of the tube. As a result of compressing the tube radially while the rollers 137 rotate around the outside surface of the tube, the tube itself has a tendency to rotate, but such rotation is prevented by the roller pairs 30-32 of roller section 8 which grip and feed the tube at a location ahead of the finning head. However, as a result of the compression of the tube against the mandrel whilethe fin forming rollers rotate around the tube, tool portion 55 also rotates, but without any detrimental effects. Where the swivel 54 is used to join tool portion 55 of the mandrel with armature portion 56 of the mandrel, the tool portion 55 can freely rotate without rotating the armature portion of the tool. This arrangement, including the swivel, can be used when necessary to reduce wear on the armature portion of the mandrel since this armature portion is not truly floating centrally of the magnets, but is usually drawn off center toward one side as shown at FIG. 4.

OPERATION of the tube and next, mandrel 44 is inserted in the tube so the several sections 46-53 of the armature are aligned respectively with the magnets 20-27. In this position the non-magnetic material portion 93 is opposite magnetic material proximity sensor 94 and tool portion 55 extends through head 113. Then, switch 108 is closed to energize the coils of magnets 20-27. Then, the rollers of roller sections 5, 6 and 8 are started to drive the tube axially through the apparatus. With head 113 rotating at approximately the required speed, cylinder 148 is actuated to move fin forming rollers 137 into engagement with the outside of the tube. Initially, the rollers will only cut a shallow thread in the surface of the tube, but will subsequently be fully cammed against the tube to form threads or fins at the full desired depth. Axial forces tending to displace tool portion 55 of the mandrel as a result of compressing the tube radially against the mandrel while moving the tube longitudinally, are resisted by the magnets 20-27. If the speed of rotation of head 113 is either slower or faster than the speed required to form threads on the tube at the rate of travel of the tube, the head will float axially so axial stresses in the fin forming elements are avoided, and there is no tendency for the elements to break. If the speed of rotation of head 113 is too great and the head tends to climb along the tube, cam 180 will move spool 177 to decrease the flow of fluid through valve 160, thereby reducing the speed of rotation of the hydraulic motor 130 which drives the head. If the speed of rotation of head 113 is too low, the head will travel with the tube, allowing spring 183 to further open the valve 160, thereby increasing the flow of hydraulic fluid from motor 130 and correspondingly, increasing the speed of rotation of the finning head. Alternatively, the speed of rotation of head 113 can be manually regulated by manipulating lever 175 to adjust valve 174 to maintain the head 113 rotating at a speed to form fins at the rate of travel of the tubing.

Should the apparatus malfunction during a tube finning operation so mandrel 44 jams and moves with the tubing, or should mandrel 44 be displaced axially from the normal position shown at FIGS. 1a and 1b without the magnets 20-27, the presence of magnetic material section 63 of the mandrel will be sensed by proximity sensor 94, and the apparatus will shut down.

As shown at FIG. 1, proximity sensor 94 controls a relay 190 with contacts 191 in the power supply lines 192 for the portions of the apparatus. As shown, motors 193, 194 and 195 are connected across the power lines. Motor 193 drives the rollers of straightening roller sections and 6, motor 194 drives the rollers of feed roller section 8, and motor 195 drives outfeed or pullout rollers 34. Solenoids 196 and 197 are also connected across the lines. Solenoid 196 maintains valve 171, which is an exhaust valve in return pipe line 172 (FIG. 5), closed so long as solenoid 196 is energized. Solenoid 197 operates an override valve 198 so connected to hydraulic cylinder 148 that the cylinder operates in its normal manner, as previously explained, so long as solenoid 197 is energized, but is extended to move fin forming rollers away from the tube 18 as soon as solenoid 197 is de-energized. Relay 190 operates to maintain contacts 191 closed so long as proximity sensor 94 senses the non-magnetic material of section 93 6 of the mandrel. If the mandrel moves so magnetic material section 93 is sensed, contacts 191 open, motors 193, 194 and 195 are de-energized, valve 171 is opened to stop motor 130, and override valve 198 is reversed to extend cylinder 148 and move the fin forming rollers 137 away from the surface of tube 18.

In series with contacts 191 of relay are contacts 200 of a microswitch 201. Microswitch 201 provides for shutting down the apparatus in the event that finning head 113 and pedestal 116 move too far axially from a predetermined central position. With reference to FIG. 6, microswitch 201 is positioned at one side of pedestal 116 and includes a roller follower 202 which engages a face 203 of a control plate 204 secured by bolts 205 to pedestal 116 for movement therewith. Plate 204 has slots 206 to provide for axial adjustment of the plate on the pedestal within predetermined limits. Face 203 is parallel with the axial direction of travel of pedestal 116. With pedestal 116 in the position shown at FIG. 6 with roller 202 engaging face 203, contacts 200 of microswitch 201 are closed and the fin forming apparatus will then operate. However, should pedestal 1 16 move too far in either direction relative to its position at FIG. 6, roller 202 will ride down onto the sloping end faces 205 of control plate 204 with the result that contacts 200 of the microswitch will open. When contacts 200 open, the circuit shown at FIG. 1a is interrupted and the apparatus is shut down in the same manner previously explained when contacts 191 of relay 190 open. By virtue of this arrangement, damage to the apparatus is prevented should mandrel 55 be displaced axially or should head 113 and pedestal 116 move too far axially. Of course, the motor for cut-off assembly 35 can also be controlled by the relay 190 and the microswitch 201.

The apparatus of this invention is capable of forming fins on relatively smaller diameter tubing, for example, 36-inch tubing, but can also be used to form fins on tubing up to 1 inch in diameter. To form fins on tubing of different diameter it is of course necessary to change portions of the apparatus. However, this changeover can be quite quickly accomplished since it is only necessary to substitute a mandrel 44 of the proper diameter, substitute pole piece sleeves and spacers having a diameter only slightly greater than the outside diameter of the tube in the magnets, change forming head 113, and adjust the nips of the rollers in roller sections 5, 6 and 8. Since the speed of rotation of finning head 113 can be varied by adjusting variable speed motor 130, the new head can be rotated at the proper speed for forming fins on the tubing, even though the thread or fin pitch is different, while maintaining the tubing feed at the previous linear speed.

While a preferred embodiment of the tube working apparatus of this invention has been shown and described in detail, and while the preferred method of practicing the invention has also been described, it is to be understood that numerous changes can be made in the preferred method and apparatus without departing from the intended scope of this invention set forth herein and defined in the appended claims.

What is claimed is:

1. Apparatus for forming fins continuously on a moving tube of indefinite length comprising in combination tube finning means adapted to engage the exterior surface of the tube to form fins thereon;

feed means to feed tubing from a source of supply continuously at a predetermined axial speed to the tube finning means;

magnet means extending along the path of travel of said tubing and closely adjacent the exterior surface of the tubing, said magnet means including a plurality of aligned, spaced apart electromagnets each comprising a coil, and

a multiplicity of spaced apart magnetic material pole pieces closely adjacent the exterior surface of said tubing;

a mandrel within said tubing having a first portion aligned with said magnet means and a second portion connected to the first portion and adjacent said tube finning means, said first portion comprising a multiplicity of magnetic material sections, and a multiplicity of non-magnetic material sections spacing said magnetic material sections from each other and joining said magnetic material sections against relative axial movement; said mandrel having its multiplicity of non-magnetic material sections longitudinally aligned respectively with said multiplicity of magnetic material pole pieces of said electromagnets; and means securing said electromagnets against axial movement; said electromagnets cooperating with said mandrel exert a strong magnetic retarding force to prevent axial movement of said mandrel during the tube finning operation. 2. Apparatus according to claim 1 wherein said apparatus further includes means fixed with respect to said magnet means and between said magnet means and tube finning means for preventing rotation of the tubing by engaging its exterior surface,

means to straighten tubing prior to feeding same to said tube finning means, and

means mounting said plurality of electromagnets in aligned end to end relation to each other along a straight section of said tubing between said finning means and said straightening means.

3. Apparatus according to claim 1 wherein said multiplicity of spaced apart magnetic material pole pieces are spaced apart by non-magnetic material spacers between said pole pieces.

4. Apparatus according to claim 1 wherein said first and second portions of said mandrel are connected together by swivel means;

whereby said second portion of said mandrel is free to rotate and tilt relative to said first portion of said mandrel.

5. Apparatus according to claim 1 whereinsaid tube finning means includes a rotating head having a tubing receiving opening therein,

a plurality of fin forming elements mounted on said head in equidistantly spaced circumferential relation to each other, and

means to simultaneously move said fin forming elements radially toward and away from tubing extending through said rotating head opening.

6. Apparatus according to claim 5 wherein bearing means mount said finning head for free movement within predetermined limits in a direction along the axis of said tube.

7. Apparatus according to claim 1 wherein said source of supply includes a coil of tubing; and

said feed means uncoils tubing from said coil, straightens the tubing, and feeds same to said tube finning means.

8. Apparatus according to claim 1 wherein said feed means feed said tube to said tube finning means at a constant linear speed;

said tube finning means comprises a rotary head, and

a plurality of fin forming elements mounted on said head in circumferentially spaced relation to each other;

and which further includes,

means mounting said head for movement along the axis of said tube; and

variable speed drive means for rotating said head at a speed such that the axial rate of formation of fins on the tube approximately equals the linear speed of tubing fed to the finning head.

9. Apparatus according to claim 8 which further includes control means responsive to movement of said head axially to adjust the speed of rotation of said drive means for said head to form fins on said tube at the rate said tubing is fed.

10. Apparatus according to claim 8 which further includes control means responsive to movement of said head axially in the direction of travel of said tubing to increase the speed of rotation of said drive means, and responsive to movement of said head axially in the other direction to decrease the speed of rotation of said drive means.

11. A method for forming fins on the exterior surface of an indefinite length of non-magnetic material tubing comprising, in combination, the steps of continuously feeding tubing along a predetermined path adjacent a fin forming station from an indefinite length supply of the tubing, said tubing having at least one open end; inserting a magnetic material mandrel into said open end of said tubing, said mandrel carrying a tool engaging the inside surface of said tubing;

positioning said mandrel in transverse alignment with a magnet station including a plurality of magnets surrounding said tube and mandrel and correspondingly positioning said tool adjacent said fin forming station;

engaging the outside surface of said tubing with a fin forming head in opposed relation to said tool; rotating said fin forming head about said tubing while feeding said tubing axially, restraining the tubing against rotation by engaging an unfinned outside surface of the tubing at a location upstream of the fin forming head, and magnetically restraining said mandrel and tool against axial movement with the tubing; and removing finned tubing at a location beyond said fin forming station.

12. A method according to claim 11 wherein said method further includes the steps of rotating said fin forming head at a speed to form helical fins on the tubing at a rate approximately equal to the linear speed of the tubing; and

feeding the tubing to said fin forming station at a predetermined linear speed.

13. A method according to claim 11 wherein said step of continuously feeding tubing includes feeding the tubing with power means;

said step of rotating said fin forming head about said tubing includes rotating said head with power means;

and which further includes sensing the axial position of said mandrel in said tubing, and

deactivating said power means in response to a sensed axial movement of said mandrel.

14. A method according to claim 11 wherein said step of removing finned tubing includes cutting said tubing into predetermined lengths while continuing feeding of the tubing.

15. Apparatus according to claim which further includes magnetically responsive sensing means outside said tubing for sensing axial movement of said mandrel; and

control means responsive to sensed axial movement of the mandrel by said sensing means for actuating said means to simultaneously move the fin forming elements away from the tubing.

16. Apparatus according to c laim which further includes additional control means responsive to sensed axial movement of the mandrel by said sensing means for stopping said feed means.

17. Apparatus according to claim 3 wherein said pole pieces and said spaces for said plurality of magnets are sleeves encircling the tubing;

each of said plurality of magnets has an opening therethrough of a diameter to receive said sleeves; and

each of said plurality of magnets includes means for releasably securing the sleeves in the magnets so that the sleeves can readily be replaced with sleeves of a different diameter to enable forming fins on tubing of a different diameter.

18. Apparatus for continuously forming a helical fin on an indefinite length of tubing comprising, in combination a source of supply of tubing;

fin forming means for forming a helical fin on the exterior surface of said tubing;

a plurality of spaced apart magnets;

feed means between said tubing supply and fin forming means for feeding tubing through the magnets and toward said fin forming means at a predetermined rate;

a cylindrical mandrel within the tubing and comprising a first portion within said magnets,

a second portion connected to the first portion in opposed relation to said fin forming means and forming a backup tool,

said first portion including a multiplicity of magnetic material sections axially spaced apart and secured together by a multiplicity of nonmagnetic material spacers;

each of said plurality of magnets includes a housing having an opening therethrough,

a flux producing coil within said housing,

a multiplicity of magnetic material pole piece sleeves spaced apart by non-magnetic material sleeves, and within the housing;

said magnets cooperating with said first portion of the mandrel to fix the mandrel against axial movement;

rotation restraining means engaging the exterior surface of the tubing at a location between said plurality of magnets and said fin forming means to prevent rotation of said tubing;

said fin forming means including a plurality of fin forming elements engaging the exterior surface of said tubing in opposed relation to said backup tool of the mandrel to roll form fins on the tubing;

means mounting said fin forming means for axial movement along the path of travel of said tubing;

drive means to rotate said fin forming means; and

control means to adjust the speed or rotation of said drive means in response to axial movement of said fin forming means, and comprising means to increase the speed of rotation of said drive means in response to movement of said fin forming means axially in the direction of travel of said tube, and

means to decrease the speed of rotation of said drive means in response to movement of said fin forming means axially in a direction opposite to the direction of travel of said tube. 

1. Apparatus for forming fins continuously on a moving tube of indefinite length comprising in combination tube finning means adapted to engage the exterior surface of the tube to form fins thereon; feed means to feed tubing from a source of supply continuously at a predetermined axial speed to the tube finning means; magnet means extending along the path of travel of said tubing and closely adjacent the exterior surface of the tubing, said magnet means including a plurality of aligned, spaced apart electromagnets each comprising a coil, and a multiplicity of spaced apart magnetic material pole pieces closely adjacent the exterior surface of said tubing; a mandrel within said tubing having a first portion aligned with said magnet means and a second portion connected to the first portion and adjacent said tube finning means, said first portion comprising a multiplicity of magnetic material sections, and a multiplicity of non-magnetic material sections spacing said magnetic material sections from each other and joining said magnetic material sections against relative axial movement; said mandrel having its multiplicity of non-magnetic material sections longitudinally aligned respectively with said multIplicity of magnetic material pole pieces of said electromagnets; and means securing said electromagnets against axial movement; said electromagnets cooperating with said mandrel exert a strong magnetic retarding force to prevent axial movement of said mandrel during the tube finning operation.
 2. Apparatus according to claim 1 wherein said apparatus further includes means fixed with respect to said magnet means and between said magnet means and tube finning means for preventing rotation of the tubing by engaging its exterior surface, means to straighten tubing prior to feeding same to said tube finning means, and means mounting said plurality of electromagnets in aligned end to end relation to each other along a straight section of said tubing between said finning means and said straightening means.
 3. Apparatus according to claim 1 wherein said multiplicity of spaced apart magnetic material pole pieces are spaced apart by non-magnetic material spacers between said pole pieces.
 4. Apparatus according to claim 1 wherein said first and second portions of said mandrel are connected together by swivel means; whereby said second portion of said mandrel is free to rotate and tilt relative to said first portion of said mandrel.
 5. Apparatus according to claim 1 wherein said tube finning means includes a rotating head having a tubing receiving opening therein, a plurality of fin forming elements mounted on said head in equidistantly spaced circumferential relation to each other, and means to simultaneously move said fin forming elements radially toward and away from tubing extending through said rotating head opening.
 6. Apparatus according to claim 5 wherein bearing means mount said finning head for free movement within predetermined limits in a direction along the axis of said tube.
 7. Apparatus according to claim 1 wherein said source of supply includes a coil of tubing; and said feed means uncoils tubing from said coil, straightens the tubing, and feeds same to said tube finning means.
 8. Apparatus according to claim 1 wherein said feed means feed said tube to said tube finning means at a constant linear speed; said tube finning means comprises a rotary head, and a plurality of fin forming elements mounted on said head in circumferentially spaced relation to each other; and which further includes, means mounting said head for movement along the axis of said tube; and variable speed drive means for rotating said head at a speed such that the axial rate of formation of fins on the tube approximately equals the linear speed of tubing fed to the finning head.
 9. Apparatus according to claim 8 which further includes control means responsive to movement of said head axially to adjust the speed of rotation of said drive means for said head to form fins on said tube at the rate said tubing is fed.
 10. Apparatus according to claim 8 which further includes control means responsive to movement of said head axially in the direction of travel of said tubing to increase the speed of rotation of said drive means, and responsive to movement of said head axially in the other direction to decrease the speed of rotation of said drive means.
 11. A method for forming fins on the exterior surface of an indefinite length of non-magnetic material tubing comprising, in combination, the steps of continuously feeding tubing along a predetermined path adjacent a fin forming station from an indefinite length supply of the tubing, said tubing having at least one open end; inserting a magnetic material mandrel into said open end of said tubing, said mandrel carrying a tool engaging the inside surface of said tubing; positioning said mandrel in transverse alignment with a magnet station including a plurality of magnets surrounding said tube and mandrel and correspondingly positioning said tool adjacent said fin forming station; Engaging the outside surface of said tubing with a fin forming head in opposed relation to said tool; rotating said fin forming head about said tubing while feeding said tubing axially, restraining the tubing against rotation by engaging an unfinned outside surface of the tubing at a location upstream of the fin forming head, and magnetically restraining said mandrel and tool against axial movement with the tubing; and removing finned tubing at a location beyond said fin forming station.
 12. A method according to claim 11 wherein said method further includes the steps of rotating said fin forming head at a speed to form helical fins on the tubing at a rate approximately equal to the linear speed of the tubing; and feeding the tubing to said fin forming station at a predetermined linear speed.
 13. A method according to claim 11 wherein said step of continuously feeding tubing includes feeding the tubing with power means; said step of rotating said fin forming head about said tubing includes rotating said head with power means; and which further includes sensing the axial position of said mandrel in said tubing, and deactivating said power means in response to a sensed axial movement of said mandrel.
 14. A method according to claim 11 wherein said step of removing finned tubing includes cutting said tubing into predetermined lengths while continuing feeding of the tubing.
 15. Apparatus according to claim 5 which further includes magnetically responsive sensing means outside said tubing for sensing axial movement of said mandrel; and control means responsive to sensed axial movement of the mandrel by said sensing means for actuating said means to simultaneously move the fin forming elements away from the tubing.
 16. Apparatus according to claim 15 which further includes additional control means responsive to sensed axial movement of the mandrel by said sensing means for stopping said feed means.
 17. Apparatus according to claim 3 wherein said pole pieces and said spaces for said plurality of magnets are sleeves encircling the tubing; each of said plurality of magnets has an opening therethrough of a diameter to receive said sleeves; and each of said plurality of magnets includes means for releasably securing the sleeves in the magnets so that the sleeves can readily be replaced with sleeves of a different diameter to enable forming fins on tubing of a different diameter.
 18. Apparatus for continuously forming a helical fin on an indefinite length of tubing comprising, in combination a source of supply of tubing; fin forming means for forming a helical fin on the exterior surface of said tubing; a plurality of spaced apart magnets; feed means between said tubing supply and fin forming means for feeding tubing through the magnets and toward said fin forming means at a predetermined rate; a cylindrical mandrel within the tubing and comprising a first portion within said magnets, a second portion connected to the first portion in opposed relation to said fin forming means and forming a backup tool, said first portion including a multiplicity of magnetic material sections axially spaced apart and secured together by a multiplicity of non-magnetic material spacers; each of said plurality of magnets includes a housing having an opening therethrough, a flux producing coil within said housing, a multiplicity of magnetic material pole piece sleeves spaced apart by non-magnetic material sleeves, and within the housing; said magnets cooperating with said first portion of the mandrel to fix the mandrel against axial movement; rotation restraining means engaging the exterior surface of the tubing at a location between said plurality of magnets and said fin forming means to prevent rotation of said tubing; said fin forming means including a plurality of fin forming elements engaging the exteriOr surface of said tubing in opposed relation to said backup tool of the mandrel to roll form fins on the tubing; means mounting said fin forming means for axial movement along the path of travel of said tubing; drive means to rotate said fin forming means; and control means to adjust the speed or rotation of said drive means in response to axial movement of said fin forming means, and comprising means to increase the speed of rotation of said drive means in response to movement of said fin forming means axially in the direction of travel of said tube, and means to decrease the speed of rotation of said drive means in response to movement of said fin forming means axially in a direction opposite to the direction of travel of said tube. 